There are premade adventures for Virtual Tabletop Software. For example the offerings at the Smiteworks store for Fantasy Grounds. But the good news is that most VTTs work by referencing a image directory containing maps and illustrations.
For PDFs it is a matter just copying the relevant map or image and pasting it into your VTT's image directory. If you don't have permission to do this with a PDF you can use the Print Screen button to capture the image and then crop it using a paint program.
For print products it is a matter of scanning the page, cropping it, and saving it into your image directory.
Finally you can use Google Image search and find thousands of images that you can use for your VTT game session including many free battlemaps.
If you are into using miniatures then you want to purchase or download tokens representing monsters and characters.
From experience teaching friends, the software and techniques to process images for VTTs are well within reach of even the most causal of computer users.
The biggest issue in my experience is not the setup of adventures but the choice of VTTs software and the setup of one's routers.
The basic issue is that some VTTs used by the referee is in reality a small server. Because of this a port needs to be opened up in your router to allow two way traffic. Fantasy Grounds and MapTools are an examples of this type of VTT. The advantage of this approach is that there is no cost beyond the initial purchase or download of the software.
The alternative is the server side VTT like Roll20. Here everybody using the webbrowser to connect to the company server to setup a session to be played. The downside that that often a ongoing subscription, often inexpensive, is required to access full functionality.
My current recommendation is to investiage Fantasy Grounds, MapTools, or Roll20 see which best suits your needs.